1. The Formation and Structure of Stars
Chapter 9
The Formation and Structure of Stars

2. The Interstellar Medium (ISM)
Gas: ~75% H, 25% He, traces of “metals”
1% “dust” (silicates, carbon, heavy elements coated with ice, About the size of the particles in smoke)
150 m average distance between dust grains
“Dense” => ~10 to 1000 atoms/cm3
“Thin” ~ 0.1 atoms/cm3

3. Structure of the ISM HI clouds: Hot intercloud medium:
The ISM occurs mainly in two types of clouds:
HI clouds:
Cold (T ~ 100 K) clouds of neutral hydrogen (HI);
moderate density (n ~ 10 – a few hundred atoms/cm3);
size: ~ 100 pc
Hot intercloud medium:
Hot (T ~ a few 1000 K), ionized hydrogen (HII);
low density (n ~ 0.1 atom/cm3);
gas can remain ionized because of very low density.

4. 3 types of nebula
Emission
Reflection
Dark
Q: Why do emission nebula look red and reflection nebula blue?

5. Evidence for the ISM
We see absorption in elements where the background stars are too hot to form these lines
Narrow width (low temperature; low density)
Multiple components (several clouds of ISM with different radial velocities)
=> Comes from the ISM

6. Interstellar reddening
Q: Why do astronomers rely heavily on IR observations?

7. Q: How do we know the ISM exists?

8. The Various Components of the Interstellar Medium
Infrared observations reveal the presence of cool, dusty gas.
X-ray observations reveal the presence of hot gas.

9. Stellar formation from the ISM:
Must be triggered by high mass stars –
Give off intense radiation
Explode as SNs
Collapsing cloud can form 10 to 1000 stars
Association
Cluster

11. The Contraction of a Protostar
Q: Why do you think there’s a lower limit on the mass of a main-seq. star?

12. The Contraction of a Protostar
Sun: ~30 million years
15 M: 160,000 years
0.2 M: 1 billion years

13. From Protostars to Stars
Star emerges from the enshrouding dust cocoon
Ignition of H  He fusion processes

14. Protostellar Disks and Jets – Herbig-Haro Objects
Q: What are the bipolar flows evidence of?
Herbig-Haro Object HH34

15. Globules Bok globules: ~ 10 – 1000 solar masses;
Bok globules:
~ 10 – 1000 solar masses;
Contracting to form protostars

16. Observations of star formation:
Evaporating gaseous globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars

17. 200 solar mass star

21. N 11B

22. V838 Mon
Trifid

23. Tarantula
N 49

25. The Source of Stellar Energy
Stars produce energy by nuclear fusion of hydrogen into helium.
Q: How does the sun fuse H to He?
In the sun, this happens primarily through the proton-proton (P-P) chain

26. The CNO Cycle Happens in stars > 1.1 M
Happens in stars > 1.1 M
More efficient that the P-P chain.
Requires high T (>16 million K)
Q: Why does the CNO require a higher temp. than the P-P chain?

27. Fusion into Heavier Elements
Fusion into elements heavier than C, O:
requires high temperatures (>600 million K);
occurs only in very massive stars (more than 8 solar masses).

28. Stellar structure Conservation of mass:
Weight of each shell = total weight
Conservation of energy:
E(out) = E(from within)
Hydrostatic equilibrium:
Pressure balances gravity
Energy transport:
Describes flow of energy

29. Hydrostatic Equilibrium
Imagine a star’s interior composed of individual shells
Within each shell, two forces have to be in equilibrium with each other:
Gravity, i.e. the weight from all layers above
Outward pressure from the interior

30. Hydrostatic Equilibrium (II)
Outward pressure force must exactly balance the weight of all layers above, everywhere in the star.
This is why we find stable stars on such a narrow strip (main sequence) in the Hertzsprung-Russell diagram.
Pressure-temperature thermostat
Q: How does the P-T thermostat control the reactions in stars?

31. Energy Transport Radiative energy transport Convection
Energy generated in the star’s center must be transported to the surface.
Inner layers of the sun:
Radiative energy transport
Outer layers of the sun (including photosphere):
Convection
Basically the same structure for all stars close to 1 solar mass.
Q: Why is convection in stars important?

33. The structure and evolution of a star is determined by the laws of
Stellar Models
The structure and evolution of a star is determined by the laws of
Hydrostatic equilibrium
Energy transport
Conservation of mass
Conservation of energy
A star’s mass (and chemical composition) completely determines its properties.
…why stars initially all line up along the main sequence, and why there’s a mass-luminosity relation….

34. The Life of Main-Sequence Stars
Stars gradually exhaust their hydrogen fuel.
They gradually becoming brighter, evolving off the zero-age main sequence (ZAMS).
Lifetime of a main-sequence star (90% of total life is on main-seq.)

35. The Lifetimes of Stars on the Main Sequence

38. The Orion Nebula: An Active Star-Forming Region

39. The Trapezium Infrared image: ~ 50 very young, cool, low-mass stars
less than 2 million years old
Infrared image: ~ 50 very young, cool, low-mass stars
X-ray image: ~ very young, hot stars
The Orion Nebula

40. Spectral types of the trapezium stars
Gas blown away from protostars
Kleinmann-Low nebula (KL): Cluster of cool, young protostars detectable only in the infrared
The Becklin-Neugebauer object (BN): Hot star, just reaching the main sequence IR
IR + visual
Spectral types of the trapezium stars B3 B1 B1 O6
Protostars with protoplanetary disks